This invention relates to a method of manufacturing a semiconductor device having a gallium nitride (GaN) film.
A gallium nitride (thereinafter, abbreviated as a GaN) semiconductor device generally has a large forbidden band energy as compared to the conventional compound semiconductor device, such as, indium phosphide (InP) and gallium arsenide (GaAs) semiconductor devices.
Therefore, a GaN based compound semiconductor which is specified by a general formula (In.sub.x Al.sub.y Ga.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1)) has been expected to be applied to a light emitter having a wavelength range between green and ultraviolet, and in particular, to a semiconductor laser having the above wavelength range.
However, there is recently no substrate which suitably matches in lattice with the GaN. Therefore, a GaN thick film is deposited on the other preliminary substrate within the range between several tens .mu.m and several hundreds .mu.m. Thus-formed GaN thick film itself is attempted to be usable as a real substrate for growing a crystal of the above GaN base compound semiconductor.
Conventionally, the above GaN thick film has been generally formed by the use of the crystal growth method using a sapphire substrate as the preliminary substrate (K. Naniwae et al.: J. Crystal Growth 99 (1990) 381, W. Gotz et al.,: Appl. Phys. Lett. 69 (1996) 242).
However, the sapphire substrate generally has a large hardness. Consequently, it is extremely difficult to polish it. Further, there is also no etching liquid enough to achieve a rapid etching rate. In consequence, after the GaN thick film is formed on the sapphire substrate, it is excessively difficult to remove the sapphire substrate from the GaN thick film in the conventional crystal growth method.
Further, a crack is generated in the GaN thick film due to the difference of the coefficient of thermal expansion and the difference of the lattice constant between the sapphire and the GaN in the conventional crystal growth method. This is mainly because the sapphire has a large hardness.
On the other hand, a semiconductor laser which is formed by a plurality of GaN based compound semiconductor layers is structured by using a quartz substrate instead of the above sapphire substrate in Japanese Unexamined Patent Publication No. Hei. 8-83928 (thereinafter, referred to as a conventional reference). In this event, although the quartz has a low hardness, the GaN based compound semiconductor is inevitably formed by a thin film. Further, crystallinity, such as, a through dislocation density, of the grown GaN based compound semiconductor layer is degraded.
Consequently, the crack brings about in the GaN based compound semiconductor layer. Moreover, cleavage planes does not match between the quartz substrate and the GaN base compound semiconductor layer. As a result, a resonator mirror surface of the laser can not be formed by the use of the cleavage of the substrate. Therefore, the mirror surface must be formed by the use of a complicated process, such as, the known dry-etching process. Further, the mirror surface has a degraded flatness.